Question

Liquid crystalline phases tend to be more viscous than the isotropic, or normal, liquid phase of the same substance. Why?

Short answer

Liquid crystalline phases have higher viscosity compared to isotropic liquid phases because of their intermediate order between liquid and solid crystalline states. The somewhat ordered arrangement of molecules in liquid crystalline phases results in stronger intermolecular forces and reinforces the molecular arrangement, making it difficult for the molecules to slide past one another. This increased resistance to flow results in higher viscosity. In contrast, isotropic liquids have weaker intermolecular forces and greater molecular mobility, resulting in lower viscosity.

Step-by-step solution

Understanding Liquid Crystals and Isotropic Liquids

Liquid crystalline phases are intermediate states between a disordered isotropic liquid and a fully ordered solid crystal. In liquid crystalline phases, the molecules have some degree of order, typically being arranged in one or more dimensions, while still maintaining a fluid-like property. On the other hand, isotropic liquids are completely disordered and display no long-range order. Their molecules move randomly in any direction in a three-dimensional space.

Molecular Interactions and Mobility

In liquid crystalline phases, the somewhat ordered arrangement of the molecules causes them to interact with each other more strongly. This results in stronger intermolecular forces as the molecules attempt to maintain their ordered positions. In isotropic liquid phases, the molecules move freely with weak interactions, due to their disordered state, and hence reduced cohesive forces.

Viscosity and Molecular Behavior

Viscosity is a measure of a fluid's resistance to flow, which can be influenced by intermolecular forces and molecular shape. A higher viscosity means the fluid is more resistant to flow and requires more force to make it flow. In the case of liquid crystalline phases, the increased order and stronger intermolecular forces make it more difficult for the molecules to slide past one another, increasing the resistance to flow. Hence, liquid crystalline phases have higher viscosity. Conversely, in isotropic liquids, the weaker intermolecular forces and random molecular arrangement allow the molecules to flow more easily, resulting in lower viscosity.

Conclusion

Liquid crystalline phases tend to be more viscous than isotropic liquid phases due to the intermediate order between liquid and solid crystalline states. This order leads to stronger intermolecular forces and reinforces the molecular arrangement, making it more difficult for the molecules to slide past one another and causing an increase in viscosity. In contrast, isotropic liquids have weaker intermolecular forces and greater molecular mobility, resulting in lower viscosity.